This invention relates to a method of recovering a metal from a metal chloride.
The invention is described hereinafter with particular reference to the recovery of silver from silver chloride. This application is however exemplary only and, where applicable, the principles of the invention can be used for the recovery of other metals such as gold and copper from their chlorides.
The miller process has been used in the refining of precious metals for a substantial period. Impurities contained in gold bullion are typically silver and base metals such as copper and nickel. Most of the impurities are removed for example by volatilization or are contained in the miller slag in the form of metal chlorides.
Normally the principal component of miller slag is AgCl with a lesser amount of Cu2Cl2. Minor amounts of Pb, Zn and Fe chlorides and sodium chloride also occur. AgCl production is also common in other precious metal refining operations used in the industry. However due to the limited, arduous, expensive and inefficient techniques which are available to convert AgCl to metallic silver, the presence of AgCl is normally regarded as undesirable.
Various techniques have been used to reduce AgCl to a metallic form.
These include the following:
    AgCl in powder form, is reduced by placing the AgCl between iron plates, adding an aqueous acid solution to the AgCl and then steam heating the mixture for two to three days to produce a silver product. Extensive washing of the silver product is required before further refining takes place;    zinc dust is added to a silver chloride slurry in an acidified environment. The zinc powder is however expensive and, again, extensive washing is required before melting of the target metal can take place;    silver chloride is reduced using soda ash, soda ash plus borax or by the addition of carbon; and    a technique described in U.S. Pat. No. 4,388,109 wherein silver chloride is mixed with sodium carbonate and heated to a temperature in excess of 500° C. to produce elemental silver and sodium chloride. Extensive washing is required before melting takes place.
It is an object of the present invention to provide an improved method for the recovery of silver wherein the metal chloride is directly reduced to metal using hydrogen gas and which is carried out at atmospheric or at a low pressure. A further object of the invention is to allow for the recovery of the hydrochloric acid gas which is evolved during the reduction process.